We propose to study the alterations that occur in protein synthesis and protein degradation during endurance training. By doing studies on intact animal, perfused muscle and in vitro muscle preparations, we hope to establish the effects of exercise on protein metabolism, elucidate the factors that mediate these effects and study the mechanisms that control the processes of protein synthesis and protein degradation. Muscle protein synthesis is depressed in trained compared to untrained rats. This effect has been demonstrated in vivo, in vitro and in a perfused muscle preparation. Muscle protein degradation is increased as a result of endurance training. Evidence for this conclusion is drawn from studies done in vivo and in the perfused muscle preparation, as well as the increase observed in 3-methylhistidine excretion in trained rats. Plasma testosterone concentrations are depressed by exhaustive exercise and endurance training. This finding may explain why urea excretion is increased in normal male rats as a result of training while there is no change in urea excretion in castrated male rats or in female rats.